The present invention relates generally to methods and apparatus for acquiring seismic data within reservoirs; and more specifically relates to methods and apparatus for improved evaluation of subsurface reservoirs, and which facilitate monitoring of formation conditions over extended time periods.
Many techniques have been proposed or utilized to evaluate subsurface formations through use of seismic data. One such technique is vertical seismic profiling (VSP), wherein one or more seismic detectors, such as geophones or hydrophones, are placed within a wellbore while a seismic source is actuated at the earth's surface. In some applications, a single seismic detector is located at different depths within the wellbore and measures energies of multiple "shots" of energy input into the earth; while in other applications multiple sensors are distributed within the well and each measures energies from a seismic shot. An analogous technique is that of reverse seismic profiling (RVSP), wherein a seismic energy source, such as for example an explosive source or airgun, is disposed within a wellbore while a plurality of seismic detectors are distributed, such as in a linear array, along the earth surface. In some cases, the energy generated by the bit has been used as the seismic source.
When VSP is utilized, the detectors respond to both upwardly propagating and downwardly propagating waves. This yields multiple arrivals and can lead to interpretational difficulties. The use of RVSP helps to minimize the presence of conflicting wave traces, in that the detectors are responsive only to upwardly propagating waves, and thus it is typically easier to define events resulting from the first arrival, and thereby to obtain improved interpretation and resolution. Both VSP and RVSP suffer from the drawback, however, that the propagating source energies must typically traverse a substantial volume of formation which is not relevant to evaluation of a region of interest. The traversing of this volume will often lead to substantial and undesirable attenuation of the source energies, thereby rendering the interpretation more difficult and adversely impacting the formation evaluation.
Another technique which has been utilized in the seismic industry is that of cross-well seismic profiling or cross-well tomography, (collectively, herein, "CSP). In cross-well analysis, a seismic source is placed in one wellbore, and one or more seismic detectors are placed in another, laterally offset, wellbore. Actuation of the seismic source generates an energy wave which propagates through a formation of interest, to yield seismic data. The generation and detecting of the seismic energies can typically be relatively localized relative to the formation of interest.
In both VSP and CSP, however, where multiple detectors are utilized, they are typically distributed generally linearly relative to the axis of the wellbore (i.e., they would be vertically arrayed in a vertical borehole). Thus, all energies are detected at the path of the borehole.
One other technique which has been utilized is "4D" seismic modeling. Such method includes the taking of three dimensional seismic data over an extended period of time; while leaving the detectors in place, either at the earth's surface or on the ocean floor. This system has limitations, however, in that it is often not practical to leave either geophones or a seismic source at a fixed location at the earth's surface (either on land or on the ocean floor), over an extended period of time. Further, when detectors are arranged in such a manner over an extended period, there is often a likelihood of movement of one or more of the detectors. Such movement may be highly detrimental to the seismic monitoring.
Prior to the present invention, available seismic surveying techniques have not provided a mechanism by which a formation could be monitored over an extended time period with detectors secured in a fixed position relative to one another. Further, it has not been possible to monitor a reservoir with downhole detectors permanently placed within the reservoir or in nearby formations. Prior art systems, therefore, have been limited in their usefulness to monitor parameters such as the condition and performance of a subsurface reservoir during the production life of that reservoir. Beyond the objective of monitoring fluid movement within the reservoir, the present system facilitates detailed stratigraphic mapping of the reservoir, and further facilitates utilizing such seismic data to provide enhanced correlation of seismic and logging data to yield more detailed definition of the reservoir.
Accordingly, the present invention provides such a new method and apparatus for acquiring seismic data subsurface formations; and further provides a system which is particularly adapted to evaluation of a reservoir over an extended time period.